Add ZeekPortWarning plugin

[awelzel]

@ckreibich - think that does it?

$ sudo ./bin/zeekctl 
WARNING: ********************************************************************************
WARNING: You're using Linux with the default ZeekPort setting 47760. This configuration
WARNING: is know to cause persistent worker failures with error messages as follows:
WARNING: 
WARNING:     error in <...>/cluster/setup-connections.zeek, lines 94-96: Failed to listen on INADDR_ANY:47764 (...)
WARNING: 
WARNING: Starting with Zeek 5.2, the default ZeekPort used by zeekctl will
WARNING: change from 47760 to 27760 in order to avoid potential port collisions
WARNING: with other processes due to 47760 falling right into Linux's default
WARNING: ephemeral port range.
WARNING: 
WARNING: Consider changing the ZeekPort option in your zeekctl.cfg to 27760
WARNING: now to prepare for this change. Doing so will silence this warning.
WARNING: 
WARNING:     ZeekPort = 27760
WARNING: 
WARNING: Note, if you're employing strict firewall rules between Zeek nodes,
WARNING: you'll likely need to update these rules. If you're using Zeek on
WARNING: a single physical host, no further action should be required.
WARNING: If possible do test the change in a non-production environment.
WARNING: 
WARNING: To silence this warning without changing the ZeekPort option,
WARNING: set zeek_port_warning.disable = 1 in zeekctl.cfg.
WARNING: 
WARNING: See the following PR for more details:
WARNING:     https://github.com/zeek/zeekctl/pull/41
WARNING: 
WARNING: Feel free to reach out on zeekorg.slack.com or community.zeek.org if
WARNING: you have any questions around this change.
WARNING: ********************************************************************************

Feel free to do any word-smithing in the PR.

[awelzel]

Placed it here: zeek/zeek#724

@timwoj - fyi, anything I can help to get this into the next 5.0.x, let me know.